Thermally excited sound wave generating device

ABSTRACT

A thermally induced sound wave generating device comprising a thermally conductive substrate, a head insulation layer formed on one surface of the substrate, and a heating element thin film formed on the heat insulation layer and in the form of an electrically driven metal film, and wherein when the heat conductivity of the thermally conductive substrate is set as α s  and its heat capacity is set as C s , and the thermal conductivity of the beat insulation layer is set as α I  and its heat capacity is set as C I , relation of 1/100≧α I C I /α S C S  and α S C S   ≧100×10   6  is realized. This is a new technical means capable of greatly improving the function of a pressure generating device based on thermal induction.

TECHNICAL FIELD

The invention of this application relates to a thermally induced soundwave generating device. More specifically, the invention of thisapplication relates to a new thermally induced sound wave generatingdevice that creates compressional wave of the air by giving heat to theair to generate sound waves and is useful for an ultrasonic soundsource, a speaker sound source, an actuator, and the like.

BACKGROUND ART

Conventionally, various ultrasonic wave generating devices have beenknown. All of these conventional ultrasonic wave generating devicesconvert some mechanical vibration into vibration of the air exceptspecial ones that use electric spark, fluid vibration, and the like. Asa method of using such mechanical vibration, although there are a movingconductor type, a capacitor type, and the like, a method utilizing apiezoelectric element is mainly used in an ultrasonic region. Forexample, electrodes are formed on both surfaces of barium titanateserving as a piezoelectric material and an ultrasonic electric signal isapplied between the electrodes, whereby mechanical vibration isgenerated and the vibration is transmitted to a medium such as the airto generate ultrasonic waves. However, in sound generating devicesutilizing such mechanical vibration, since the sound generating deviceshave inherent resonance frequencies to the sound generating devices,there arc problems in that frequency bands are narrow, the soundgenerating devices are susceptible to influences of an ambientenvironment (temperature, vibration) and the like, and it is difficultto fine and array the sound generating devices.

On the other hand, a pressure wave generating device based on a newgeneration principle, which does not involve mechanical vibration atall, has been proposed (JP-A-11-300274) (Nature 400 (1999) 853-855). Inthis proposal, specifically, the pressure wave generating deviceincludes a substrate, a heat insulation layer provided on the substrate,and a heating element thin film that is provided on the heat insulationlayer and driven electrically. By providing the heat insulation layersuch as a porous layer or a polymeric layer having extremely smallthermal conductivity for heat generated from the heating element thinfilm, a temperature change in an air layer on the surface of a heatingclement is increased to generate ultrasonic sounds. Since, the proposeddevice does not involve mechanical vibration, the device hascharacteristics that a frequency band is wide, the device is lesssusceptible to influences of an ambient environment, and it isrelatively easy to fine and array the device. Considering a generationprinciple for such a pressure generating device based on thermalinduction, a change in surface temperature at the time when an ACcurrent is applied to the electrically-driven heating element thin filmis given by the following expression (1) when thermal conductivity ofthe heat insulation layer is set as α, a heat capacity per volumethereof is set as C, and an angular frequency thereof is set as ω, andthere is output and input of energy per a unit area of q(ω)[W/cm²].T(ω)=(1−j){square root}{square root over (2)}×1/{square root}{squareroot over (ωαC)}× q(ω)  (1)

In addition, a sound pressure generated at that point is given by thefollowing expression (2).P(ω)=A×1{square root}{square root over (αC)}×q(ω)  (2)

In short, as shown in FIG. 5, a temperature change of the air is caused(FIG. 5-c) by heat exchange of heat (Fig. 5-b), which is generated fromthe heating element thin film by an electric current (FIG. 5-a) with afrequency f supplied from a signal source for generating a signal of anultrasonic frequency, with the air that is a medium around the heatingelement thin film. This generates a compressional wave of the air,whereby a sound wave with a frequency 2 f is generated (FIG. 5-d).

Here, it is seen from the expression (2) that the sound pressure to begenerated is larger as the thermal conductivity α and the heat capacityper volume C of the thermal insulation layer are smaller, and isproportional to the output and input q(ω) of energy per a unit area,that is, input electric power. Moreover, thermal contrast of the heatinsulation layer and the substrate plays an important role. When athickness of the heat insulation layer having the thermal conductivity αand the heat capacity per volume C is set as L and there is a thermallyconducive substrate having sufficiently large α and C below the heatinsulation layer, if the heat insulation layer has a thickness (athermal diffusion length) of a degree represented by the followingexpression (3),L=(2α/ωC)^(0.5)  (3)it is possible to insulate an AC component of generated heat and permitheat of a DC component, which is generated because of a heat capacity ofthe heating element, to escape to the substrate having the large thermalconductivity efficiently.

However, in the sound wave generating device based on thermal induction,under the present situation, no actual prospects are opened up from theviewpoint of improvement in performance thereof concerning an issue ofhow a multilayer structure thereof should be and concerning a specificform thereof. Although the sound wave generating device does not involvemechanical vibration at all and has many characteristics, there is aproblem in that, when it is attempted to obtain practical output, Jouleheat generated by an increase in input power also increases due toincrease of input power, it is impossible to permit heat of a DCcomponent to escape completely, and it is impossible to increase atemperature change in the heating element thin film.

A level of a sound pressure to be generated is about 0.1 Pa at themaximum, which is not a satisfactory level. Therefore, furtherimprovement in the performance has been desired.

Thus, it is an object of the invention of this application to providenew technical means that can realize significant improvement inperformance for a pressure generating device based on thermal inductionthat does not involve mechanical vibration and has many characteristics.

DISCLOSURE OF THE INVENTION

Firstly, the invention of this application provides, as a device forsolving the problems, a thermally induced sound wave generating deviceincluding: a thermally conductive substrate; a heat insulation layerformed on one surface of the substrate; and a heating element thin filmformed on the heat insulation layer and in the form of an electricallydriven metal film, and wherein when thermal conductivity of thethermally conductive substrate is set as α_(s) and a heat capacitythereof is set as C_(s), and thermal conductivity of the heat insulationlayer is set as α_(I) and its capacity is set as C_(I), relation of1/100≧α_(I)C_(I)/α_(S)C_(S) and α_(S)C_(S)≧100×10⁶ is realized.

Secondly, the invention provides the thermally induced sound wavegenerating device that is characterized in that the thermally conductivesubstrate consists of a semiconductor or metal. Thirdly, the inventionprovides the thermally induced sound wave generating device that ischaracterized in that tile thermally conductive substrate consists of aceramics substrate.

As described above, the inventors repeated studies earnestly payingattention to thermal contrast of the heat insulation layer and thesubstrate in order to solve the problems and, as a result of thestudies, the invention of this application is derived. The invention iscompleted on the basis of a totally unexpected new knowledge thatperformance is improved by selecting materials for the thermallyconductive substrate and the heat insulation layer such that therelation described above is realized.

Fourthly, the invention of this application provides the thermallyinduced sound wave generating device that is characterized in that theheat insulation layer is a porous silicon layer that is formed on onesurface of the thermally conductive substrate by anodizingpolycrystalline silicon. Fifthly, the invention provides the thermallyinduced sound wave generating device that is characterized in that theporous silicon layer has silicon grains of a columnar structure at leastin a part in the porous silicon layer.

As described above, the invention is derived from the result of theearnest studies by the inventors and is completed on the basis of atotally unexpected new knowledge that, by using the porous siliconlayer, which is formed by making polycrystalline silicon porous, as thebeat insulation layer, a part of the porous silicon layer plays a roleof permitting heat of a DC component to escape to the substrate sideefficiently.

Sixthly, the invention of this application provides the thermallyinduced sound wave generating device that is characterized in that, inthe porous silicon layer, dielectric films are formed on surfaces ofnanocrystalline silicon. Seventhly, the invention provides the thermallyinduced sound wave generating device, characterized in that thedielectric films are oxide films. Eighthly, the invention provides thethermally induced sound wave generating device that is characterized inthat the dielectric films are nitride films. Ninthly, the inventionprovides the thermally induced sound wave generating device that ischaracterized in that the dielectric films are formed according to heattreatment. Tenthly, the invention provides the thermally induced soundwave generating device that is characterized in that the dielectricfilms are formed according to electrochemical treatment.

The inventors repeated studies earnestly in order to solve the problemsand, as a result of the studies, these inventions are completed on thebasis of a totally unexpected new knowledge that, in a thermally inducedsound generating device that is characterized by including: a thermallyconductive substrate; a heat insulation layer consisting of a poroussilicon layer that is formed on one surface on the substrate; and aheating element thin film consisting of a metal film that is formed onthe heat insulation layer and driven electrically, it is possible todecrease thermal conductivity a in a heat insulation layer and it ispossible to increase a generated sound pressure by forming dielectricfilms on surfaces of nanocrystalline silicon of the porous siliconlayer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating an embodiment mode of athermally induced sound wave generating device according to theinvention of this application

FIG. 2 is a diagram showing a preferred range for a relation betweenα_(S)C_(S) and α_(I)C_(I).

FIG. 3 is a schematic sectional view showing a columnar structure ofsilicon grains.

FIG. 4 is a schematic sectional view showing a state in which dielectricfilms are formed on surfaces of nanocrystalline silicon.

FIG. 5 is a diagram showing a relation among a frequency, an electriccurrent, beat, temperature, and a sound wave.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention of this application has the characteristics as describedabove. An embodiment mode of the invention will be hereinafterexplained.

FIG. 1 is a sectional view illustrating an embodiment mode of athermally induced sound wave generating device according to theinvention of this application. In an example of FIG. 1, the thermallyinduced sound wave generating device includes: a thermally conductivesubstrate (1), a heat insulation layer (2) consisting of a poroussilicon layer that is formed on one surface of the substrate, and abeating element thin film (3) consisting of a metal film that is formedon the heat insulation layer (2) and driven electrically.

When a thickness of a thermally insulating layer having thermalconductivity c and a heat capacity per volume C is set to L and there isa thermally conductive substrate having sufficiently large α and C belowthe thermally insulating layer, if the heat insulation layer has athickness (a thermal diffusion length) of a degree represented by theexpression (3), It is possible to insulate an AC component of generatedheat and permit heat of a DC component, which is generated because of aheat capacity of a heating element, to escape to the substrate havinglarge thermal conductivity.

In order to make a flow of this heat more efficient, as shown in FIG. 2,materials for the heat insulation layer and the substrate are selectedand combined such that α_(I)C_(I) is within a range of1/100α_(I)C_(I)/α_(S)C_(S) and α_(S)C_(S)≧100×10⁶. Here, when thematerials are combined under a condition of 1/100<α_(I)C_(I)/α_(S)C_(S)and/or α_(S)C_(S)<100×10⁶, it is impossible to permit the heat of the DCcomponent to escape to the substrate side sufficiently and heataccumulates in the heating element metal thin film. Thus, it isimpossible to obtain a sufficient temperature change with respect toinput and the characteristics of the thermally induced sound wavegenerating device are deteriorated. In addition, although a lower limitof a value of α_(I)C_(I)/α_(S)C_(S) and an upper limit of α_(S)C_(S) arenot specifically provided, practical limits are values of a combinationof metal and a high performance heat insulating material that havelargest contrast.

αC values of various materials are listed specifically in Table 1. TABLE1 Thermal conductivity α, Heat capacity C. Thermal conductivity α Heatcapacity C. Type (W/mK) (10⁶ J/m³K) αC (×10⁶) Copper 398 3.5 1393Silicon 168 1.67 286 Al₂O₃ 30 3.1 93 SiO₂ 1.4 2.27 3.2 Polyimide 0.161.6 0.26 Porous silicon 0.12 0.5 0.06 Polystyrene 0.04 0.045 0.0018 foam

αC of a solid body generally takes values in ranges indicated In Table 1in cases of metal, a semiconductor, an inorganic insulator, and resin.Here, the porous silicon is a porous body of silicon that can be formedby, for example, subjecting a silicon surface to anodic oxidationtreatment in a hydrogen fluoride solution. It is possible to obtain adesired porosity and a desired depth (thickness) by appropriatelysetting an electric current density and treatment time. The poroussilicon is a porous material and shows extremely small values in boththermal conductivity and a heat capacity compared with silicon accordingto a quantum effect (a phonon confinement effect) of nano-sized silicon.

More specifically, it is seen from Table 1 that, for example, whencopper or silicon is used as the substrate, the polyimide, the poroussilicon, the polystyrene foam, and the like can be used as the heatinsulation layer. The combination of these heat insulating materials isonly an example and a combination of heat insulating materials can beselected appropriately. However, preferably, heat insulating materials,from which the heat insulation layers can be manufactured in an easymanufacturing process such as fibing/arraying treatment, are selected.

As described above, it is possible to obtain the heat insulation layer(2) consisting of the porous silicon layer by subjecting the siliconsurface to the anodic oxidation treatment in a hydrogen fluoridesolution. In that case, it is possible to obtain a desired porosity anda desired depth (thickness) by appropriately setting an electric currentdensity and treatment time. The porous silicon is a porous material andshows extremely small values in both thermal conductivity and a heatcapacity compared with silicon according to a quantum effect (a phononconfinement effect) of nano-sized silicon. More specifically, whereasthe silicon has the thermal conductivity α of 168 W/mK and the heatcapacity C of 1.67×10⁶J/m³K, the porous silicon with a porosity of about70% has the thermal conductivity α of 0.12 W/mK and the heat capacity Cof 0.06×10⁶J/m³K.

As the silicon, it is possible to use polycrystalline silicon ratherthan single crystalline silicon. The polycrystalline silicon can beformed by, for example, the plasma CVD method. However, a method offormation is not specifically limited. The polycrystalline silicon maybe formed according to the catalyst CVD method or may be obtained byforming a film of amorphous silicon according to the plasma CVD methodand, then, applying laser anneal to the amorphous silicon film asheating treatment to thereby polycrystallize the amorphous silicon film.When the polycrystalline silicon is treated according to the anodicoxidation method, as shown in FIG. 3, it is possible to form a porousstructure (2-b) in which fine columnar structures (2-a), which areaggregates of grains (crystal particles), are present and siliconnano-sized silicon crystals are present among the fine columnarstructures. It is considered that this is because an anodic oxidationreaction of the polycrystalline silicon progresses preferentially inboundaries of the grains, that is, anodic oxidation progresses in adepth direction among columns of the columnar structure, and thecolumnar silicon grains still remain even after the anodic oxidation. Byadopting such a structure, it is possible to permit heat to escape tothe substrate side efficiently in the part of the columnar structurewhile maintaining a macroscopic function as the beat insulation layer.

It is needless to mention that a size and a rate per a unit volume ofpresence of the silicon grains of this columnar structure changedepending on conditions of the anodic oxidation. In the invention ofthis application, such presence of the silicon grain is presented as amore preferable form.

In addition, the inventors of this application paid attention to thefact that thermal conductivity of SiO₂ and Si₃N₄, which were insulatingmaterials, was small compared with thermal conductivity of the siliconthat was a skeleton of the porous silicon. In short, as shown in FIG. 4,the inventors found that it was possible to reduce the thermalconductivity α of the porous silicon by forming dielectric films onsurfaces of nanocrystalline silicon forming the porous silicon anddecreasing thermal conductivity of the skeleton portions. However, sinceheat capacities C of these insulating materials is large compared withthat of the silicon, it-is necessary to appropriately select a thicknessof the dielectric films to be formed on the surfaces of the siliconcrystals such that the αC value are small.

Although a method of forming these dielectric films is not specificallylimited, it is preferable to form the dielectric films according to, forexample, heat treatment or electrochemical treatment. It is possible toperform the heat treatment by applying heat under an oxygen atmosphereor a nitrogen atmosphere. A temperature condition, a temperature risecondition, and the like at that point are selected appropriatelydepending on a material of a substrate to be used or the like. Forexample, it is possible to perform thermal oxidation treatment in atemperature range of 800° C. to 950° C. for 0.5 to 5 hours. It ispossible to perform the electrochemical oxidation treatment by feeding aconstant current between the substrate and a counter electrode for apredetermined time in an electrolyte solution such as a sulfuric acidaqueous solution. It is possible to select a current value, a conductingtime, and the like at that point appropriately according to a thicknessof an oxide film desired to be formed.

As the thermally conductive substrate (1), in order to permit heat of aDC component to escape, it is preferable to use a material having largethermal conductivity α and it is most preferable to use metal. Forexample, substrates having high thermal conductivity of copper andaluminum are selected. However, the substrate (1) is not limited tothese, and it is possible to use a semiconductor substrate such as asilicon substrate. In addition, it is also possible to use a ceramicsubstrate such as glass. As a form of the substrate, a beat radiationfin may be formed on a rear surface thereof in order to improve heatradiation efficiency.

Next, a material for the heating element thin film (3) is notspecifically limited as long as the heating element thin film (3) is ametal film. For example, single metal such as W, Mo, Ir, Au, Al, Ni, Ti,or Pt or a laminated structure of these pieces of metal is % used. It ispossible to form the heating element thin film (3) according to vacuumevaporation, sputtering, or the like. In addition, it is preferable tomake a thickness of the heating element thin film (3) as small aspossible in order to reduce a heat capacity. However, it is possible toselect the thickness in a range of 10 nm to 100 nm in order to have anappropriate resistance.

Thus, embodiments will be described below to explain the invention ofthis application more in detail. It is needless to mention that theinvention is not limited by the following embodiments. Embodiments

First Embodiment

A film of Al was formed 300 nm as a contact electrode for anodicoxidation treatment on a rear surface of a P-type (100) singlecrystalline silicon substrate (80 to 120 Ωcm) (α_(S)C_(S)=286×10⁶)according to vacuum evaporation. Thereafter, this substrate wassubjected to the anodic oxidation treatment at a current density of 100mA/cm² for eight minutes with platinum as a counter electrode in asolution of HF(55%):EtOH-1:1 to form a porous silicon layer(α_(I)C_(I)=0.06×10⁶) with a thickness of about 50 μm. Finally, W wasformed in a thickness of 50 nm as a heating element thin film on theporous silicon layer according to the sputtering method to manufacturean element with an area of 5 mm².

Second Embodiment

A layer (α_(I)C_(I)=0.26×10⁶) coated with polyimide in a thickness of 50μm was formed on an upper surface of a substrate of pure copper(thickness 1 mm) (α_(S)C_(S)=1393×10⁶). Finally, W was formed in athickness of 50 nm as a heating element thin film on the polyimideaccording to the sputtering method to manufacture an element with anarea of 5 mm².

Third Embodiment

An SiO₂ layer (α_(I)C_(I)=3.2×10⁶) with a thickness of 2 μm was formedon an upper surface of a substrate of pure copper (thickness 1 mm)(α_(S)C_(S)=1393×10⁶) according to the sputtering method. Finally, W wasformed in a thickness of 50 nm as a heating element thin film on theSiO₂ according to the sputtering method to manufacture an element withan area of 5 mm².

FIRST COMPARATIVE EXAMPLE

An Al₂O₃ film (α_(I)C_(I)=93×10⁶) with a thickness of 2 μm was formed onan upper surface of a P type (100) single crystalline silicon substrate(80 to 120 Ωcm) (α_(S)C_(S)=286×10⁶) according to the sputtering method.Finally, W was formed in a thickness of 50 nm as a heating element thinfilm on the Al₂O₃ film according to the sputtering method to manufacturean element with an area of 5 mm².

SECOND COMPARATIVE EXAMPLE

A layer (α_(I)C_(I)=0.0018×10⁶) coated with polystyrene foam in athickness of 100 μm was formed on an upper surface of soda glass(α_(S)C_(S)=1393×10⁶) with a thickness of 1.1 mm. Finally, W was formedin a thickness of 50 nm as a heating element thin film on thepolystyrene foam according to the sputtering method to manufacture anelement with an area of 5 mm².

Electric power of 50 kHz and 1 W/cm² was supplied to the heating elementthin films of the elements obtained in the first to the thirdembodiments and the first and the second comparative examples to measureoutput sound pressures with a microphone at a distance of 10 mm from theelements.

A result of the measurement is shown in Table 2. TABLE 2 Heat insulationα_(s)C_(s) Output sound No. Substrate layer α₁C₁/α_(s)C_(s) (×10⁶)pressure (Pa) First embodiment Silicon Porous silicon 1/4764 280 0.28Second embodiment Copper Polyimide 1/5358 1393 0.17 Third embodimentCopper SiO₂ 1/435 1393 0.11 First comparative Silicon Al₂O₃ 1/3.1 2800.01 example Second comparative SIO₂ Polystyrene 1/1778 3.2 0.03 examplefoam

Ultrasonic waves of 100 kHz were generated from the respective elementsof the first to the third embodiments and the first and the secondcomparative examples. It is seen from Table 2 that a sound pressureincreases for a combination of 1/100≧α_(I)C_(I)/α_(S)C_(S) andα_(S)C_(S)≧100×10⁶.

Fourth Embodiment

A film of polycrystalline silicon was formed in a thickness of 3 μm on asurface of a substrate of pure copper with a thickness of 1 mm accordingto the plasma CVD method.

Thereafter, this substrate was subjected to the anodic oxidationtreatment at a current density of 20 mA/cm² for three minutes withplatinum as a counter electrode in a solution of HF(SS %):EtOH=1:1 toform a porous silicon layer. Finally, W was formed in a thickness of 50nm as a heating element thin film on the porous silicon layer accordingto the sputtering method to manufacture an element with an area of 5mm². When the porous silicon layer of the obtained element was observed,a columnar structure of silicon grains was observed. Moreover, electricpower of 50 kHz and 50 W/cm² was supplied to the heating element thinfilm of the obtained element to measure an output sound pressure with amicrophone at a distance of 10 mm from the element. As a result,generation of an ultrasonic wave of 100 kHz was confirmed and the soundpressure output was 5.8 Pa. A steady-state temperature on the surface ofthe element at that point was about 50° C.

THIRD COMPARATIVE EXAMPLE

A film of Al was formed 300 nm as a contact electrode for anodicoxidation treatment on a rear surface of a P-type (100) singlecrystalline silicon substrate (3 to 20 Ωcm) according to vacuumevaporation. Thereafter, this substrate was subjected to the anodicoxidation treatment at a current density of 20 mA/cm² for three minuteswith platinum as a counter electrode in a solution of HF(55%):EtOH=1:1to form a porous silicon layer with a thickness of about 3 μm. Finally,W was formed in a thickness of 50 nm as a heating element thin film onthe porous silicon layer according to the sputtering method tomanufacture an element with an area of 5 mm². When the porous siliconlayer of the obtained clement was observed, a columnar structure ofsilicon grains was not observed specifically. Moreover, electric powerof 50 kHz and 50 W/cm² was supplied to the heating element thin film ofthe obtained element to measure an output sound pressure with amicrophone at a distance of 10 mm from the element. As a result,generation of an ultrasonic wave of 100 kHz was confirmed and the soundpressure output was 3.8 Pa. A steady-state temperature on the surface ofthe element at that point was about 80° C.

It was also confirmed from the above that, In the thermally inducedsound wave generating device according to the invention of thisapplication, by using the porous silicon layer, which was formed bymaking polycrystalline silicon porous, as the heat insulation layer,since that portion permits heat of a DC component to escape to thesubstrate side efficiently, it was possible to generate sound wavesefficiently even for high power output.

Fifth Embodiment

A film of Al was formed 300 nm as a contact electrode for anodicoxidation treatment on a rear surface of a P-type (100) singlecrystalline silicon substrate (3 to 20 Ωcm) according to vacuumevaporation. Thereafter, this substrate was subjected to the anodicoxidation treatment at a current density of 20 mA/cm² for forty minuteswith platinum as a counter electrode in a solution of HF(55%):EtOH=1:1to form a porous silicon layer with a thickness of about 50 μm.Thereafter, the substrate was subjected to the thermal oxidationtreatment at 900° C. for ten minutes in an oxygen atmosphere to formdielectric films consisting of SiO₂ on surfaces of nanocrystallinesilicon. Finally, W was formed in a thickness of 50 nm as a heatingelement thin film on the porous silicon layer according to thesputtering method to manufacture an element with an area of 5 mm².

Sixth Embodiment

An element was manufactured in the same manner as the fifth embodimentexcept that the treatment was performed in a nitrogen atmosphere as heattreatment to form a dielectric film consisting of Si₂N₄.

Seventh Embodiment

An element was manufacture in the same manner as the fifth embodimentexcept that the electrochemical oxidation treatment was performed toform a dielectric film consisting of SiO₂. More specifically, thetreatment was performed at a current density of 5 mA/cm² for 10 minuteswith a platinum electrode as a counter electrode in a 1M sulfuric acidaqueous solution.

FOURTH COMPARATIVE EXAMPLE

An element was manufactured in the same manner as the fifth embodimentexcept that the thermal oxidation treatment was not performed.

The thermal conductivity at and the heat capacity C of the poroussilicon layer were measured for the fifth to the seventh embodiments andthe fourth comparative example according to an photo-acoustic method. Inaddition, electric power of 50 kHz and 1 W/cm² was supplied to theheating element thin films of the obtained elements to measure outputsound pressures with a microphone at a distance of 10 mm from theelements.

A result of the measurement is shown in Table 3. TABLE 3 Thermal Outputconductivity Heat sound α capacity C. pressure No. (W/mk) (10⁶ J/m³K) αC(×10⁶) (Pa) Fifth embodiment 0.1 1.2 0.12 0.25 Sixth embodiment 0.3 1.30.39 0.14 Seventh embodiment 0.1 1.1 0.11 0.26 Fourth comparative 1.10.7 0.77 0.10 example

Ultrasonic waves of 100 kHz were generated from the respective elements.From Table 3, by forming the dielectric layer, although the heatcapacity C increases slightly, the thermal conductivity decreases and,as a result, a value of αC decreases. Therefore, the output soundpressure to be generated increased.

Consequently, in the thermally induced sound wave generating deviceaccording to the invention of this application, in the thermally inducedsound wave generating device including the thermally conductivesubstrate, the heat insulation layer consisting of the porous siliconlayer formed on one surface on the substrate, and the heating elementthin film consisting of a metal film that is formed on the heatinsulation layer and driven electrically, by forming the insulating filmon the surfaces of the silicon crystals of the porous silicon layer, itis possible to decrease the thermal conductivity a in the heatinsulation layer and it is possible to increase a generated soundpressure.

INDUSTRIAL APPLICABILITY

As described above in detail, in the thermally induced sound wavegenerating device according to the invention of this application, thethermally induced sound wave generating device includes: the thermallyconductive substrate; the heat insulation layer formed on one surface ofthe substrate; and the beating element thin film consisting of a metalfilm that is formed on the heat insulation layer and drivenelectrically, and, when thermal conductivity of the thermally conductivesubstrate is set as α_(s), a heat capacity thereof is set as C_(s),thermal conductivity of the heat insulation layer is set as α_(I), and aheat capacity thereof is set as C_(I), materials for the thermallyconductive substrate and the heat insulation layer are selected suchthat a relation of 1/100≧α_(I)C_(I)/α_(S)C_(S) and α_(S)C_(S)≧100×10⁶ isrealized. Consequently, it is possible to improve an output soundpressure characteristic significantly.

In addition, in the thermally induced sound wave generating deviceaccording to the invention of this application, the porous siliconlayer, which is formed by making polycrystalline silicon porous, is usedas the heat insulation layer. Consequently, since the silicon grains ofthe columnar structure permit heat of a DC component to escape to thesubstrate side efficiently, it is possible to generate sound wavesefficiently even for high power output.

Further, in the thermally induced sound wave generating device accordingto the invention of this application, in the thermally induced soundgenerating device including; the thermally conductive substrate; theheat insulation layer consisting of the porous silicon layer that isformed on one surface on the substrate; and the heating element thinfilm consisting of a metal film that is formed on the heat insulationlayer and driven electrically, dielectric films are formed on surfacesof nanocrystalline silicon of the porous silicon layer. Consequently, itis possible to decrease thermal conductivity α in a heat insulationlayer and it is possible to increase a generated sound pressure.

1. A thermally induced sound wave generating device comprising: athermally conductive substrate; a heat insulation layer formed on onesurface of the substrate; and a heating element thin film formed on theheat insulation layer and in the form of an electrically driven metalfilm, and wherein when thermal conductivity of the thermally conductivesubstrate is set as α_(s) and its heat capacity is set as c_(s), andthermal conductivity of the heat insulation layer is set as α_(I) andits heat capacity is set as c_(I), relation of1/00≧α_(I)C_(I)/α_(S)C_(S) and α_(S)C_(S)≧100×10⁶ is realized.
 2. Athermally induced sound wave generating device according to claim 1,characterized in that the thermally conductive substrate consists of asemiconductor or metal.
 3. A thermally induced sound wave generatingdevice according to claim 1, characterized in that the thermallyconductive substrate consists of a ceramics substrate.
 4. A thermallyinduced sound wave generating device according to claim 1, characterizedin that the heat insulation layer is a porous silicon layer that isformed on one surface of the thermally conductive substrate by makingpolycrystalline silicon porous.
 5. A thermally induced sound wavegenerating device according to claim 4, characterized in that the poroussilicon layer has silicon grains of a columnar structure at least in apart in the porous silicon layer.
 6. A thermally induced sound wavegenerating device according to claim 4 or 5, characterized in that, inthe porous silicon layer, dielectric films are formed on surfaces ofnanocrystalline silicon.
 7. A thermally induced sound wave generatingdevice according to claim 6, characterized in that the dielectric filmsare oxide films.
 8. A thermally induced sound wave generating deviceaccording to claim 6, characterized in that the dielectric films arenitride films.
 9. A thermally induced sound wave generating deviceaccording to claim 6, characterized in that the dielectric films areformed according to heat treatment.
 10. A thermally induced sound wavegenerating device according to claim 6, characterized in that thedielectric films are formed according to electrochemical treatment. 11.A thermally induced sound wave generating device according to claim 5,characterized in that, in the porous silicon layer, dielectric films areformed on surfaces of nanocrystalline silicon.
 12. A thermally inducedsound wave generating device according to claim 7, characterized in thatthe dielectric films are formed according to heat treatment.
 13. Athermally induced sound wave generating device according to claim 8,characterized in that the dielectric films are formed according to heattreatment.
 14. A thermally induced sound wave generating deviceaccording to claim 9, characterized in that the dielectric films areformed according to heat treatment.
 15. A thermally induced sound wavegenerating device according to claim 7, characterized in that thedielectric films are formed according to electrochemical treatment. 16.A thermally induced sound wave generating device according to claim 8,characterized in that the dielectric films are formed according toelectrochemical treatment.
 17. A thermally induced sound wave generatingdevice according to claim 9, characterized in that the dielectric filmsare formed according to electrochemical treatment.